The present invention relates to a disk drive actuator system, and more particularly to an integrated programmable error amplifier for controlling a current signal used to drive an actuator to position a transducing head.
Disk drive systems conventionally use an actuator arm to move a transducing head to a radial position on a magnetic disk. A position control loop is used to read the actual radial position of the head and compare it to the desired position, which generates a position error signal. This position error signal is used to generate a command to move the head to the correct position.
In order to move the head, a servo controller is provided to convert the position error signal generated by a position control loop into a current command for controlling a current control loop to drive the disk drive actuator motor to move the head. The current control loop includes an error amplifier which compares a desired actuator motor current to an actual actuator motor current, and controls the current delivered to the actuator motor accordingly. The error amplifier is typically configured with a single ended operational amplifier as part of an actuator motor controller on an integrated circuit, with three discrete resistors and two discrete capacitors. The error amplifier is implemented to have a pole in its response at a frequency of zero, and also to have a zero in its response. The zero in the response of the error amplifier is created by one of the resistors and one of the capacitors. Often a second pole is also desirable in the response of the amplifier/controller, to improve the rise time of the response and to improve noise performance. The second pole is created by the same resistor that creates the zero in the response, and by the two capacitors.
While the error amplifier configured with a single ended operational amplifier has provided effective response characteristics for controlling the position of the head, the necessity for discrete components requires extra pins in the integrated circuit package, which can increase packaging costs, and requires additional printed circuit board space to implement the discrete components externally, which is undesirable since space is at a premium for the increasingly small disk drives currently in production. These problems would be magnified even further if the circuit were to be built differentially, which could improve the power supply rejection ratio (PSRR) of the circuit. There is a need in the art for an error amplifier and compensation circuit that may be realized in an integrated circuit while providing the high performance response characteristics needed to effectively control the current provided to the actuator motor to position the head in the disk drive system.
The present invention is an error amplifier for use in a disk drive actuator system having an actuator motor for radially positioning a transducing head with respect to a rotatable disk. The error amplifier includes a switched-capacitor proportional-integral controller for comparing first and second differential input signals representing a respective actual current and commanded current for driving the actuator motor, and providing a differential output signal. A transconductor-capacitor filter is connected to filter the differential output signal of the switched-capacitor proportional-integral controller, and provides a motor control signal to control the actuator motor.